A heterojunction bipolar transistor (hereinafter, abbreviated as HBT) has been known as a typical example of a group III-V semiconductor device (see “III-V ZOKU KAGOBUTU HANDOTAI”, authored by Isamu AKAZAKI, Baifukan, published on May 20, 1994, first edition, pp. 239-242). The HBT is a type of semiconductor device (semiconductor element) serving as a signal amplifier for use in a high frequency band such as a microwave band. The HBT is a semiconductor element provided with an NPN-type or a PNP-type heterojunction structure, comprising three functional layers such as an emitter layer for substantially supplying (emitting) an element operating current or electrons, a base layer, and a collector layer for collecting the element operating current (“HANDOTAI KOGAKU”, edited by Toyoshi FUKAMI, Tokyo Denki Daigaku Shuppan Kyoku, published on Mar. 20, 1993, 1st edition, 7th printing, pp. 97-99). According to the structure of the element, the base layer is interposed between the emitter layer and the collector layer (see the aforementioned “HANDOTAI KOGAKU”, pp. 97-99).
In an HBT employing a group III-V compound semiconductor, a high carrier (hole) concentration p-type group III-V semiconductor layer is employed as the base layer, which has an important function of regulating element operation current flowing between the emitter and the collector (see the aforementioned “HANDOTAI KOGAKU”, pp. 239-242). In an NPN-type or a PNP-type HBT employing a compound semiconductor formed of a material system selected from an aluminum gallium arsenide (AlxGa1-xAs; 0<x<1)/gallium arsenide (GaAs)-system and a gallium indium phosphide (GayIn1-yP; 0<y<1)/GaAs-system, a p-type base layer is produced from gallium arsenide. In an NPN-type gallium indium arsenide-system HBT, an n-type Ga0.51In0.49P emitter layer is formed on the front surface of a p-type GaAs base layer and an n-type GaAs collector layer is formed on the rear surface of the p-type GaAs base layer (see IEEE Inc. 21st Annual GaAs IC symposium (Oct. 177-20, 1999), Technical Digest 1999, pp. 237-240).
As a p-type base layer, a crystal layer having low resistance, i.e., a carrier (hole) concentration greater than approximately 1×1019 cm−3, is formed by a vapor growth method such as MOCVD (metal-organic chemical vapor deposition). In vapor growth, a conventional method has been known for vapor growing a p-type GaAs vapor growth layer having a high carrier concentration by doping an acceptor impurity such as zinc (Zn) (see Electron. Lett., 29 (1993), p. 1725). The other conventional method has also been known for vapor growing a p-type GaAs vapor growth having a high carrier concentration by doping carbon (see Applied Phys. Lett., 68 (7) (1996), pp. 982-984). Carbon (C) is generally used for growing the p-type GaAs layer having a high carrier concentration because of ease of producing high hole concentration and because of lower diffusion property than those of group II acceptor impurities (see J. Vac. Sci. technol. B, 14 (6) (1996), pp. 3509-3513).
In order to form a carbon-doped p-type GaAs layer through a method such as MOCVD, the following methods can be employed:
(1) a method employing pyrolysis of an organic arsenic compound such as trimethylarsine ((CH3)3As) (see Abstract of Fourth European Workshop on Metalorganic Vapor Phase Epitaxy (5-7 Jun. 1991, Nijmegen, The Netherlands), PROGRAM and ABSTRACT, Poster);
(2) a method employing carbon tetrachloride (CCl4), carbon tetrabromide (CBr4), or derivatives thereof serving as carbon sources (see J. Electron. Mater., Vol. 29., No. 2 (2000) pp. 205-209 and Appl. Phys. Lett., 62 (11) (1993) pp. 1248-1250); and
(3) a method in which an arsenic (As) source for MOCVD growth of GaAs, such as arsine (AsH3), and agallium (Ga) source such as trimethylgallium ((CH3)3Ga) are supplied to a vapor phase reaction system as a As source/Ga source ratio (i.e., V/Ill ratio) that is controlled to a low level (see the aforementioned J. Electron. Mater., Vol. 29, No. 2).
However, conventional HBTs employing a carbon-doped GaAs-system crystal layer having a high carrier (hole) concentration of carbon (C) have been known to have a problem in that element flow current (emitter-collector current) disadvantageously changes over time (see “2000 GaAs MANTECH Conference (May 1-4, 2000)”, Digest of Papers (GaAs Mantech, Inc., USA 2000), pp. 131-135). For example, when current drift is generated, the ratio (β) (β=Ic/Ib) of the collector current (Ic) to base current (Ib); i.e., current gain (see “DENSHI DEBAISU KOGAKU”, authored by Seijiro FURUKAWA et al., Morikita Shuppan K. K., published on Oct. 16, 1995, 1st edition, 8th printing, pp. 62-63) becomes unstable disadvantageously.
The present invention has been accomplished on the basis of the finding that the aforementioned current gain and current drift depend on the optical property of the carbon-doped p-type GaAs-system crystal layer and on the concentration of hydrogen serving as an impurity and remaining in the crystal layer. In particular, an object of the present invention is to provide a carbon-doped GaAs crystal layer which effectively reduces current drift in the conventional group III-V compound semiconductor HBTs. Another object of the present invention is to provide group III-V compound semiconductor HBTs which include pn junction structure or semiconductor heterojunction structure of the group III-V semiconductor and a carbon-doped GaAs-system crystal layer.